Open cell vitreous carbon foam, also called reticulated vitreous carbon foam, is a vitreous carbon structure of three dimensional interconnected polyhedron cells. Vitreous carbon is a glassy carbon as opposed to a graphitic carbon. A cell is typically comprised of 12 to 14 connected struts or ligaments which form a three dimensional polyhedron. An open cell is a cell where no film or solid is connected between the ligaments so that there is free communication in to and out of a cell, and in a group of adjoining cells all cells are interconnected and form a three dimensional interconnected structure. An open cell foam has a structure which typically consists of three dimensional interconnected polyhedron cells with 12 to 14 ligaments composing each cell, and each cell having a spheroidal shape.
The degree to which the cell shape approaches a perfectly spherical shape is measured by the aspect ratio. The aspect ratio is a dimensionless number, defined for the purpose of this patent application, as the diameter of the cell in the vertical direction divided by the diameter of the cell in the horizontal direction. A perfectly spherical cell would has an aspect ratio of 1.
The shape of the cell and hence the aspect ratio of the cell can effect the performance of the bulk vitreous carbon foam which is comprised of these cells. The strength of the bulk vitreous carbon and the resistance to flow of fluids through the bulk vitreous carbon are some of the physical properties of the bulk carbon foam affected by the aspect ratio.
Since the physical properties of carbon foam are affected by the aspect ratio, an aspect ratio of 1 would be required for a completely isotropic foam. Users of carbon foam often desire isotropic behavior from the foam and therefore desire a foam with an aspect ratio as close to 1 as possible, and find an aspect ratio between 0.6 to 1.2 acceptable for these purposes.
Vitreous carbon foam is often coated with other materials via a chemical vapor deposition process. If the underlying vitreous carbon foam has a vast majority of cells with aspect ratios greatly different than 1 the resultant coated foam will have cells with aspect ratios greatly different than 1 and the resultant coated foam may have anisotropic properties. Chemical vapor deposition is used to coat carbon foams with ceramics such as SiC or BN and metals such as W or Ta. One application of Ta coat carbon foam is in vivo exoskeleton bone implants and tissue reception as shown by Kaplan U.S. Pat. No. 5,282,861. In this particular application it has been found necessary to produce carbon foam billets at least 6" thick and as much as 8" thick where the cells of the carbon foam have an aspect ratio between 0.6 to 1.2. Therefore there is a demonstrated commercial need for vitreous carbon foam of thickness greater than 6" having cells with aspect ratios between 0.6 to 1.2.